This invention relates to a hard, gold-colored underlayer for a gold or gold-containing surface layer, and article therewith.
Decorative articles such as writing implements, cigarette lighters, spectacle frames, watch cases and straps, and ornaments of all types having a gold or gold-containing surface layer are desired, but it is also desired to keep the amount of gold per unit area of the surface layer as small as possible on cost grounds. Processes have long since been developed, therefore, for producing thin surface layers of gold or gold-containing material on such articles. Because the surface layers are thin, however, there is risk of the surface layers prematurely wearing away from the articles, especially at their projecting parts. Depending upon the types of demands placed on the article, mechanical and/or chemical wear can cause this, for example by rubbing or perspiration.
It is known from German patent publication OS No. 2,825,513 to provide an underlayer of titanium nitride (TiN) beneath a gold or gold-containing surface layer which, because of its general gold color, tends to disguise any wearing away of the surface layer. It is also known from the same publication that the gold color of the underlayer and the surface layer can be influenced for adaptation to one another by admixing or alloying other materials with one or both layers.
It has been shown, however, that titanium nitride cannot itself produce a realistic gold color as is apparent, also, from the diagram of the referenced publication. It has been shown, further, that titanium nitride, even with admixture of further metals for a realistic gold color, does not have the "brilliance" of gold, but is more matte. Matte surfaces generally look darker so that any parts of such an underlayer that show through the surface layer appear darker on account of lacking brilliance even though the underlayer has the same color as the surface layer.
For example, by measurements with a color measuring apparatus of the MacBeth 1500 type using a light source of type C, the so-called CIELAB units of color and brilliance of materials can be determined. Details of this measuring process are given later. An L* value in CIELAB units characterizes the brilliance. The L* value for different gold alloys lies above 85 and represents a high brilliance. The known underlayer of TiN, however, has a L* value of only 73. This represents a much-reduced brilliance as compared to gold alloys, noting that a very matte material does not have a zero L* value, but one between about 40 and 50.
Experiments have been carried out to increase the brilliance (L* value) of titanium nitride by altering the stoichiometry of the compound. Thus, for example, the compound TiN.sub.x, wherein x is less than 1, gives a brighter gold color as compared to stoichiometric titanium nitride, TiN. The increase in the L* (brilliance) value is, however, relatively trivial, to 77, and is accompanied by the disadvantage that the gold color of the TiN.sub.x exhibits a green tint. The green tint is undesirable for color compatibility with gold-colored surface layers having a red color component because, after the surface layer erodes, there is an obvious difference in the color of the underlying layer.
Hardness or durability is also required for a wear-disguising underlayer like that described in the referenced publication so that, when the surface layer has worn away to expose the underlayer, the underlayer itself does not wear away too quickly and, thereby, destroy its wear-disguising function.